1. Field of the Invention
The present invention relates to a cataptric telephoto lens, and more particularly to a catadioptric telephoto lens.
2. Description of the Prior Art
A catadioptric telephoto lens is generally composed of four components; first being a positive lens member positioned in the front most part of the lens system for converging the incident light beam; second being a similarly converging main mirror member positioned with a large air gap in the image field side of the first positive lens member; third being a diverging subsidiary mirror member positioned in the vicinity of the first positive lens member; and fourth being a diverging negative lens group positioned in the vicinity of the main mirror member. Such structure is being widely employed because of various advantages such as a small total and a reduced color aberration.
Such conventional catadioptric telephoto lens is disclosed for example in the U.S. Pat. No. 3,632,190 corresponding to the Japanese Patent Publication No. 1909/1972, the Japanese Patent Publication No. 30288/1979, and the U.S. Pat. No. 4,188,091 corresponding to the Japanese Patent Laid-open No. 13183/1978. Though such lens is far shorter than the dioptric telephoto lenses, it is not yet satisfactory as the total length from the frontmost lens surface to the focal plane is larger than 30% of the focal length and the shortest focusable distance is larger than 8 times of the focal length.
The Japanese Patent Laid-open No. 38520/1980 discloses a structure allowing reduction of the entire length to 28% of the focal length by positioning a subsidiary mirror member at the object side of the positive lens member. However, the mirror member is generally supported by adhesion to the positive lens member. Consequently, the lens surface of the positive lens member, though transmitting the light beam twice through the reflection on said subsidiary mirror member, cannot be designed independently and has a reduced freedom for correcting the aberrations so that it often becomes difficult to achieve satisfactory correction. The subsidiary mirror member receives the converging light beam from the main mirror member, while an annular light beam entering the positive lens member passes around said subsidiary mirror member, so that a shielding tube has to be provided around said mirror member for dividing said light beams and preventing flare formation by the direct entry to the image plane of the light beam entering the positive lens member. However, the positioning of said shield tube becomes difficult when the subsidiary mirror member is provided at the object side of the positive lens member, so that the elimination of ghost light becomes difficult. In addition, when the subsidiary mirror member is positioned in front of the positive lens member, it is difficult to protect the subsidiary mirror member from external shock, so that the lens becomes less durable.